A hairspring is a key component of a mechanical timepiece. It is one of the two main components of an oscillator, the other being the balance wheel. The oscillator of a timepiece provides the means of time regulation via its simple harmonic motion. The balance wheel acts as the inertial element and is attached to the inner terminal of a spiral-shaped hairspring. The outer terminal of the hairspring is typically rigidly attached to a fixed stud. In an ideal oscillator, the hairspring provides a restoring torque to the balance wheel that is proportional to the wheel's angular displacement from an equilibrium position, and equations of motion describes this as a linear second-order system.
The equilibrium position is defined as the angular position of the balance wheel such that when the oscillator is static, the net torque applied by the hairspring on the balance wheel is zero. An oscillator is considered isochronous when its natural frequency is independent of its amplitude and other external factors, such as temperature variations, magnetic fields and the like. As the accuracy of the timepiece is largely determined by the stability of the oscillator's natural frequency, isochronism is one of the most important properties of a mechanical timepiece.
Historically, a hairspring has been considered one of the most challenging components to manufacture for a timepiece, in particular in respect of mechanical movements used in watches. It is required to flex continuously at a frequency typically from three to five Hertz, this being the frequency range of a modern day mechanical timepiece oscillator, for the entire lifespan of the timepiece before maintenance, typically several years. A hairspring is also one of the smallest components in a mechanical movement having a spiral strip width of typically in the range of from 30 to 40 microns.
A hairspring also has to be formed from materials that resist the effect of temperature variations on the material properties, especially the Young's modulus, so as to maintain correct time-keeping and minimize fluctuations.
Furthermore and with the increasing amount of electrical and magnetic fields due to the proliferation of consumer electronics, a modern-day hairspring also must be able to resist or substantially minimize the effect of magnetic fields thereon. This is due to accuracy and consistency of a hairspring's stiffness being a demanding parameter, whereby even a 0.1% stiffness variation can result in up to 1 minutes/day in timepiece inaccuracy, which would be unacceptable in the timepiece and watch industries, and those in the watch industry have historically expended much effort with a view to providing hairsprings which minimize such effects by way of design and manufacturing technology.
Traditional hairsprings are made of metal alloys beginning from hardened steel used by John Harrison nearly 300 years ago to Elinvar invented by Charles Guillaume in 1919 to most recently Nivarox invented by Dr. H. C. Reinhard Straumann. Nearly all modern hairsprings are made of some variation of Nivarox, an alloy based on iron and nickel. The hairsprings are manufactured in a drawing process where the strip material is drawn into a thin wire. The straight strips are then coiled into a spiral before being treated to stabilize the shape of the spiral. This process has several known disadvantages including:                (a) A drawing process is typically not a high precision technique, having a tolerance in the range of a few microns, a significant percentage of the hairspring strip width typically in the range of 30 to 40 microns, resulting in inconsistency of stiffness,        (b) Metal alloys such as Nivarox inherently have a tendency to creep and deform slightly after prolonged stress in use, such that the metal hairspring cannot maintain its original spiral shape after over a year of continuous operation, which may require adjustment and inevitably affects time-keeping accuracy, and        (c) Although the thermal elastic constant and magnetic sensitivity of materials Nivarox have been substantially reduced compared with earlier metallic hairsprings by heavy doping with trace elements such as chromium, these problems and shortcomings have not been completely eliminated.        
With a view to address or minimise the aforementioned problems with Nivarox and other metal alloys and their manufacturing methods for hairspring, the past decade has seen the introduction of the use of silicon and micro-fabrication techniques in hairspring manufacture.
Silicon hairsprings are produced using the micro-fabrication process which can achieve sub-micron precision with significantly greater accuracy than conventional metal forming techniques such as drawing. Advantages of the use of silicon include the following:                (a) Such a material does not creep and oxidize over time in comparison with most metal alloys, thus maintaining mechanical properties and integrity,        (b) Such a material is entirely non-magnetic, and        (c) Temperature sensitivity may be minimized or substantially eliminated for normal operating parameters, by provision of a hairspring with a silicon core and a thin layer of silicon dioxide such that the net thermal elastic constant of a hairspring approaches zero.        
The use of technology of fabricating silicon hairspring accordingly has had several advancements over the past decade including that as disclosed in document DE 10127733 (7 Jun. 2001) whereby it is disclosed the use of silicon micromechanical springs, in which mono-crystalline silicon, which is in the <100> or <111> plane, whereby both orientations are disclosed as being equally suitable. The spring is a spiral spring having a good resistance to large-scale thermal stresses as well as good stability in shape. A coating of silicon dioxide can also cover the springs as disclosed.
Document EP 1422436 (25 Jun. 2002) describes a method to reduce the thermal drift of a single spiral hairspring for a timepiece, so as to achieve a temperature coefficient approaching zero. The method and device use a spiral spring intended to equip the balance of a mechanical timepiece and is formed of a rod of the spiral cutting from a <100> mono-crystalline silicon wafer having first and second order thermal elastic constants, the turns of the coil spring having a width w and a thickness t, whereby a coating of silicon dioxide making it possible to minimize the thermal coefficients of the spring constant of the spiral spring. The spiral spring described thus ideally comprises a modulation of the width of the spring.
Document EP2224293 (29 Apr. 2004) discloses a timepiece movement comprising a regulating device comprising a balance wheel and a plane hairspring which may be formed from silicon. The plane hairspring comprising on its outer turn a stiffened portion arranged to cause the deformations of the turns to be substantially concentric. The stiffened portion ends before the outer end of the hairspring, and characterised in that the between a terminal portion of the outer turn and the last-but-one turn of the hairspring is large enough for said last-but-one turn to remain free radially during expansions of the hairspring up to amplitudes corresponding substantially to the maximum angle of rotation of the balance in the movement. This assists in maintaining the concentricity of the hairspring when in use, thus assisting in maintaining good timekeeping. It is disclosed that the silicon hairspring may be formed using the method of EP 0732635. This patent, in the name of Patek Philippe, is known to disclose the structure of its Spiromax® balance spring, as discussed below.
In 2006, Patek Philippe publicly released the Spiromax® balance spring made of Silinvar®. This hairspring is obtained by a vacuum oxidation process that allows it to compensate for temperature variations. The concentric nature (the symmetrical expansions and contractions of the balance spring in relation to its centre) is made possible by a terminal curve that is not turned up but rather has a noticeably thicker region at the outer end, as described and claimed in EP 2224293.
Document EP 2215531 (28 Nov. 2007) describes a mechanical oscillator for watchmaking, comprising a spiral spring formed from a mono-crystalline silicon (Si) is oriented along the crystallographic axis <111> which has a coating is selected to obtain a variation, as a function of the temperature, of the resilient torque of the spiral spring, compensating the variation in function of the temperature at the moment of inertia of the balance. This document uses a mono-crystalline silicon material of axis <111> in the same manner as <001> mono-crystalline silicon is used in EP 1422436 and a coating to provide a temperature insensitive hairspring, also in the manner that the Spiromax hairspring is formed from Silinvar with an oxide coating to provide a hairspring which is insensitive to temperature changes.
More recent silicon hairsprings have also been available, however such use has been limited commercially, and in view of the limited usage of silicon hairsprings for a significantly short period of time of less than a decade and not in widespread usage in comparison to metal alloy hairsprings which have been used prolifically for many decades, the long-term reliability and integrity of hairsprings formed from silicon has not as yet had the opportunity to be assessed and compared with the industry standard metal or metal alloy hairsprings such as the Nivorax metal alloy hairsprings.